The recent rapid expansion of wireless communications has increased the desire for radio type communications circuits. The increase in cellular telephone sales/services is a prime example of expansion.
Many wireless communications circuits use phase splitters. For example, digital wireless telephone signals are transmitted at high frequencies. When a signal is received by a digital wireless telephone, the high frequency signal must be converted to a baseband signal from which bits of digital information, e.g., voice information, may be recovered. To do this, quadrature phase splitters ("splitters"), which are defined below, are used, in combination with mixers, to reduce the lowest intermediate frequency signals to the baseband signal.
Splitters are designed to receive an input signal and produce two output signals. Splitters are designed such that, for a center frequency, the two output signals are 90 degrees out of phase with each other (i.e., are in a quadrature relation with each other) and have the same magnitude. In practice, there is a range of frequencies, whose midpoint is the center frequency, for which the two output signals are considered to be in a "quadrature relation" and have the "same" magnitude. This is became some error (e.g., 89.5 degrees out of phase, magnitudes within one percent of each other, etc. . . . ) is tolerable. Thus, we will refer to the center frequency and the range of frequencies around the center frequency for which there are tolerable errors as, collectively, the center frequency.
Phase splitters may be designed with discrete components. Such a design could be implemented with two resistors ("R") and two capacitors ("C") arranged to form a high pass RC filter and a low pass RC filter. Designers could choose values for R and C such that the two output signals have equal magnitudes and are in a quadrature relation at the desired center frequency (centered about f=1/2.pi.RC). If the two output signals are to exhibit these characteristics, the Rs must be the same value and the Cs must be the same value.
However, electronic manufacturers, in their desire to manufacture smaller electronic products, prefer to have circuits, including phase splitters, contained within ICs. When any circuit, including a phase splitter, is implemented on an IC, the value of, e.g., all Rs can be made the same but the value cannot be preselected due the manufacturing process of ICs. The same is true of the value for all Cs. Further, the values for R and C may change with temperature depending upon the environment in which the electronic product containing the IC is operated. These factors result in the inability of the designer to specify the desired center frequency at which the two output signals will have both equal magnitudes and be in a quadrature relation.
In attempting to solve this problem, "A 2 V 2 GHz Si-Bipolar Direct-Conversion Quadrature Modulator" by Tsukaham et at, 1994 IEEE ISSCC, pages 40-41, describes a phase splitter that uses reverse biased diodes ("diodes") instead of capacitors. Unlike capacitors, the diodes can be externally tuned. In this case, the diodes are tuned with external DC voltages, VCI and VCQ, as shown in FIG. 3 of the reference. The need for external tuning is undesirable for a number of reasons. First, external tuning adds to the manufacturing cost of the IC. Second, externally tuned ICs are temperature sensitive. Additionally, another problem associated with using diodes is that the diodes do not exhibit the linear behavior of a capacitor. The value of a capacitor varies in proportion to the inverse of the capacitor's dielectric thickness, which is a constant thickness for a true capacitor. However, for the diodes, the dielectric thickness varies with the voltage across the device. Thus, in operation, the voltage across the diode includes an external tuning voltage (e.g., VCI and/or VCQ) and a sinusoidal signal riding on top of the external timing voltage. The sinusoidal signal varies the dielectric thickness of each of the two diodes, Q1 and Q2, giving rise to their nonlinear behavior. Because of the nonlinear behavior, even though the magnitudes of the two output signals are equal, the two output signals are no longer in a quadrature relation.
Other attempts have been made. "A 0.7-3 GHz GaAs QPSK/QAM Direct Modulator", by A. Boveda et at., Digest of Technical Papers, 1993 IEEE ISSCC, pages 142-143, discloses a splitter that uses variable resistive elements, in the form of junction field effect transistors ("JFETs") that can be externally tuned such that the two output signals are in quadrature with each other. JFETs are highly nonlinear. Thus, even though the JFETs can be externally tuned such that the two output signals are in a quadrature relation, the two output signals will no longer be of equal magnitude.
It is desirable to implement a phase splitter circuit on a single IC that exhibits little or no nonlinearity and does not have to be externally tuned while providing two output signals which, for a desired center frequency, are in a quadrature relation and have equal magnitudes.